Shells which are fired from a conventional mortar achieve a maximum range of about 8 km. These shells are unguided and therefore have a fairly large spread in respect of a target.
By providing such a shell with wings and a guiding device, the range of fire can be considerably extended, at the same time as the impact accuracy is improved. Shells which are arranged with extensible wings for increased range of fire have long been known. Patent specification U.S. Pat. No. 130,078, FIG. 1, shows a shell having extensible wings 11, which wings are extended directly after firing from a launcher. The wings lie retracted against the outer side of the shell body and are pretensioned with a spring mechanism, which is activated mechanically after the shell has passed through the launcher.
By virtue of patent specification WO9846962 A1, it is also known to use time-delayed extension of the wings on a shell or missile. The wings of the shell can thus be extended, for example, only once the shell has reached its maximum flying height or summit of trajectory, in order subsequently to glide in the downward part of the flight path.
The time delay can be pre-programmed or activated via an internal GPS system. The time delay can also be remote-controlled from a ground-based target radar, with account being taken of changed target conditions, or if shells are fired in sequence.
Small wings give limited gliding ratio with relatively high trajectory speed, which gives moderately long trajectory times. The gliding ratio is the ratio between the flight distance ahead and the reduction in flight height. Typically, the gliding ratio for winged shells or missiles lies in the order of magnitude of 3-6. Large airliners have a gliding ratio in the order of magnitude of 15, and extreme gliders have a gliding ratio above 60. For a shell having wings which are extended at the summit of trajectory of the shell for gliding toward a target, a gliding ratio of about 4 implies an extended range of fire from about 8 km to about 16 km compared with a wingless shell.
Guidance of conventional aircraft can be easily described such that a rudder in the tail of the aircraft makes deviations which change the angle of incidence of the aircraft and thus of the wing to the flowing air. A change in the angle of incidence of the wing changes the lift of the aircraft and thus the flight path of the aircraft. The rudder can be placed in front of the wing or behind the wing. It is harder to achieve good flight stability, however, when the rudder is placed in front of the wing, so that a configuration is normally chosen in which the rudder is placed in the tail of the aircraft. In patent specification U.S. Pat. No. 5,780,760, a mortar shell having extensible gliding wings and a guiding device arranged in the tail of the shell are described.
For a mortar shell, it is impractical to have a guiding device, such as, for example, fins, arranged in the tail of the shell, since the fins are subjected to high pressures and temperatures from the burning propellant powder charge of the shell. Fins are therefore normally arranged in the nose of the mortar shell.
The drawback with fins in the nose of the shell is, however, limited guiding facilities, resulting in worse flight stability, as well as a complicated construction due to double sets of wings.